Photoplethysmography (PPG) is a non-invasive optical technique for monitoring variation in blood volume or blood flow near the surface of the skin to determine various physiological parameters. The principle uses an illumination source and a photodetector to measure changes in intensity as light is passed through or reflected from body tissue. The detected optical signals are then analysed and correlated with the pulsation of blood through the body, as stimulated by the heartbeat. More background information about PPG can be found in the topical review “Photoplethysmography and its application in clinical physiological measurement”, John Allen, Physiol. Meas. 28 (2007) R1-R39, as well as in the following papers co-written by the present inventors and their colleagues:
“Non-contact Reflection Photoplethysmography towards Effective Human Physiological Monitoring”, Shi, P., Azorin Peris, V., Echiadis, A., Zheng, J., Zhu, Y., Cheang, P. Y. S., Hu, S.*, J Med. Biol. Eng. Vol.30 Iss 30 (2010): 161-167;
“Insight into the dicrotic notch in photoplethysmographic pulses from the finger tip of young adults”, Shi, P., Hu, S.*, Zhu, Y., Zheng, J., Qiu, Y., Cheang, P. Y. S., J Med Eng Tech, Vol. 31, Iss 8 (2009), 628-633;
“Analysis of pulse rate variability derived from photoplethysmography with the combination of lagged Poincaré plots and spectral characteristics”, Shi, P., Zhu Y., Allen, J., Hu, S.*, Med Eng. Phys. 31 (2009) 866-871, doi: 10. 1016/j.medengphy.2009.05.001;
“A Preliminary Attempt to Understand Compatibility of Photoplethysmographic Pulse Rate Variability with Electrocardiogramic Heart Rate Variability”, Shi, P., Hu, S., Zhu, Y., J Med. Biolog. Eng., Vol.28, Iss 4 (2008): 173-180;
“A Monte Carlo Platform for the Optical Modelling of Pulse Oximetry”, Azorin-Peris, V., Hu, S.*, Smith, P. R, Applied Physics Letters, SPIE, Vol.6446 (2007), pp.64460T;
“An Effective Solution to Reduce Motion Artefact in New Generation Reflectance Pulse Oximeter”, Alzahrani, A., Hu, S., SIECPC-Apr 2013;
“Electrically conductive adhesive enable to manufacture high performance patch probe for non-invasive physiological assessment”, Zhang, X., Hu, S.*, Liu, C., Azorin-Peris, V., Imms, R Proc Electronics System-Integration Tech Conf (ESTC) 2012;
“A study of opto-physiological modelling to quantify tissue absorbance in imaging photoplethysmography”, Hu, S.*, Azorin Penis V., Zheng J., Proc. IEEE EMBC 32, 2010, 1: 5776-5779; and
“Development of effective photoplethysmographic measurement techniques: from contact to non-contact and from point to imaging”, Hu, S.*, Azorin Penis V., Echiadis A., Zheng J., and Shi P., Proc. IEEE EMBC 31, 2009, 1: 6550-6553.
In an earlier application, WO2009/030934 (incorporated herein by reference), the applicants describe a method of forming an image comprising sequentially illuminating a target with a plurality of illumination pulses and consolidating the image pulse signals to provide a real time image of blood pulsing, which is dependent upon blood oxygen saturation within a blood circulation system.
Opto-physiological technology (including PPG) is applicable to the global mobile health market, which is growing rapidly in developed countries in response to the rising incidence of lifestyle induced chronic diseases and ageing populations. The two main applications of this technology are: i) self-monitoring, for example, for sports performance or to maintain or improve general wellbeing and fitness levels; and ii) clinician-monitoring to capture important health parameters of chronically ill patients or those undergoing post-operative care. In addition, monitoring may be performed to identify or confirm underlying illnesses or to monitor the vital parameters of at-risk patients to track underlying conditions and provide an early warning signal in order to prevent exacerbation.
In relation to current PPG systems, pulse oximetry devices (which determine blood oxygen saturation) are most common. However, these are often inaccurate as they do not take into account the light scattering effects of live tissue, motion induced artefacts or the effects of sweat, skin creams or sprays.
In the consumer fitness market, electrical chest-strap based continuous heart rate monitoring systems are often employed. However, these tend to be uncomfortable and obtrusive making them unpopular for frequent use. Other wearable monitoring systems are emerging in the market, including wristwatch-based sensors that can measure individual parameters such as heart rate and/or respiratory rate. However, no wearable sensor is currently available that can simultaneously measure a wider range of physiological parameters, including blood oxygen saturation.
It is therefore an aim of the present invention to provide an opto-physiological (OP) sensor and a method of assembling the same, which helps to address the afore-mentioned problems.